1. Field of the Invention
The present invention relates to turbochargers and more particularly to a turbocharger compressor rotor housing.
2. Prior Art
Power output of a naturally aspirated internal combustion engine may be significantly increased by the addition of a turbocharger. Turbochargers include a compressor, generally of the positive-displacement or dynamic type, for providing an air or air fuel charge at a greater than ambient pressure and density to the combustion chamber. The turbocharger normally includes a turbine, driven by exhaust gases from the turbocharged engine for powering the compressor.
Centrifugal flow compressors are one of the most widely used dynamic compressors in turbochargers. In this compressor type, air or an air-fuel mixture enters the compressor inlet, is channeled to the compressor rotor and is accelerated to near sonic speeds at a right angle to the inlet flow path. Increase in air pressure is accomplished by reducing the velocity of the accelerated gases as discharged from the tip of the compressor rotor blades. This process, known as diffusion, is more efficiently achieved by slowing down the gases without turbulence so that a large percentage of the velocity energy is converted into pressure energy, raising the static pressure.
To facilitate this diffusion process, turbochargers employing centrifugal compressors have normally included a compressor rotor wall closely following the contour of the compressor rotor blades from the blade leading edge to its outer tip. This compressor wall extends past the outer tip of the blade, then terminates to provide a circumferential gap through which the compressed gases are channeled into a chamber leading to the intake manifold of the engine. This wall, facing the compressor rotor blades and closely contoured to the rotor blades, uniformly decreases the velocity of the gases after the gases leave the rotor blades and prior to their entry into the chamber leading to the engine. Thus, this wall structure greatly increases the static pressure generated by the compressor.
To form this structure, most turbocharger compressor housings have been sand cast with the compressor wall cast in one piece with the compressor outer surround housing. This has normally been accomplished by using a sand core to form the circumferential chamber leading to the intake manifold of the engine. After casting, this sand core is dislodged to produce the chamber on the opposite side of the wall from the compressor in which gases are channeled off of the tips of the compressor rotor.
Although die-casting of the compressor housing would be substantially less expensive and more accurate than sand castings, die-casting of an optimum design has not been possible because of the inability to use die-cast molds to form the circumferential chamber which channels the compressed gases to the intake manifold of the engine and at the same time form the diffuser wall. Because the variable area chamber is necessarily larger than the inlet gap through which gases are injected from the compressor rotor blades, die-casting an optimum design compressor housing has not been possible because of the inability to design molds that would form this passageway behind the wall facing the blades of the compressor rotor.
Where die-cast compressor housings are used, the wall normally formed in sand cast compressor housings is merely deleted so that the molds may be brought together and parted to form the casting. However, without this wall, gases accelerated by the compressor rotor are prematurely dumped from the compressor rotor blades into the circumferential chamber leading into the engine intake manifold. As a result, this arrangement realizes a substantially lower compressor efficiency and thus lower performance.